The present invention relates to a process for preparing asymmetric poly(4-methyl-pentene-1)(hereinafter referred to as TPX) porous membranes for gas separation, and in particular to a process for preparing asymmetric TPX porous membranes for gas separation by wet phase and dry/wet inversion method.
Several materials, such as poly(4-methyl-pentene-1) (TPX), polysulfone (PSf), polymethyl siloxane, polyimide, acetic acid cellulose, and polycarbonate, have been used in the preparation of porous membranes for gas separation, especially oxygen-nitrogen separation. Among them, TPX, due to its excellent mechanical strength, high oxygen/nitrogen selectivity and fairly high gas permeability, has been commercialized. Taking the X-22, a TPX oxygen-nitrogen separation membrane supplied by Mitsui Co. Ltd, Japan, for example, its permeability ratio of O.sub.2 /N.sub.2 can reach 4.15 and the oxygen permeability thereof is 0.61 GPU(1 GPU=1.times.10.sup.-6 cm.sup.3 /cm.sup.2 .sup.x sec.times.cm Hg). However, there is still a need to further increase the gas permeability while retaining permeability ratio of O.sub.2 /N.sub.2 so as to increase the commercial value of the TPX separation membranes. To improve the gas permeability of TPX membranes, composite and/or grafted TPX membranes have been developed for gas separation. For example, in J. Appl. Polym. Sci., 32, 5763 (1986), there is disclosed a 4-vinylpyridine .gamma.-ray irradiation-grafted TPX membrane. Its oxygen permeability is 25.4 Barrer (1 Barrer=1.times.10.sup.-10 cm.sup.3 .times.cm/cm.sup.2 .times.sec.times.cm Hg), and the permeability ratio of O.sub.2 /N.sub.2 is 11.1. In J. Appl. Polym. Sci., 34 559(1987), a TPX/siloxane blend membrane for oxygen enrichment is disclosed. The obtained membranes have an oxygen permeability of 57.4 Barrer and a permeability ratio of O.sub.2 /N.sub.2 of 7.81.
Recently, due to the process complexity and the high material cost for the production of composite and/or grafted TPX membranes, a wet phase inversion method has been developed for the preparation of asymmetric porous membranes for gas separation. A polymer solution is cast on a suitable support and immersed in a nonsolvent coagulation medium. The asymmetric membrane is formed by exchange of solvent and coagulation medium and the resulting structure is a formed membrane having an integral skin layer suitable for gas separation and a porous base layer having low permeation resistance. For example, in J. Appl. Polym. Sci., 43, 1491 (1991),a PSf asymmetric porous membrane was prepared and was reported to have improved gas permeability. In J. Membr. Sci., 86, 103(1994) and Eur. Polym. J. 30, 883(1994), PC asymmetric membranes having high gas permeability have been prepared by wet phase inversion method by respectively adding additive nonsolvent alcohols and oxygenphilic transition metal salts to the casting solution.
TPX membranes for gas separation have not yet been prepared by wet phase inversion method.